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Study of TCP Performance over Mobile Networks

Study of TCP Performance over Mobile Networks. Outline. Problems with TCP Class of solutions Review some of the proposals Snoop – TCP I – TCP End To End Protocols References. Traditional TCP. Assume congestion to be the primary cause for packet losses and unusual delays

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Study of TCP Performance over Mobile Networks

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  1. Study of TCP Performance over Mobile Networks

  2. Outline • Problems with TCP • Class of solutions • Review some of the proposals • Snoop – TCP • I – TCP • End To End Protocols • References

  3. Traditional TCP • Assume congestion to be the primary cause for packet losses and unusual delays • Invoke congestion control and avoidance algorithms, resulting in significant degraded end-to-end performance and very high interactive delays

  4. Wireless Networks • Communication characterized by • sporadic high bit-error rates (10-4 to 10-6) • disconnections • intermittent connectivity due to handoffs • low bandwidth

  5. FH BS1 BS2 MH MH Mobile Networks Topology FH – Fixed Host BS – Base Station MH – Mobile Host

  6. TCP Performance with BER “HALA ELAARAG” - “Improving TCP Performance over Mobile Networks” – “ACM Computing Surveys, Vol. 34, N0 3, Sep 2002, pp 357-374”

  7. Classification of Schemes • End-to-End protocols • loss recovery handled by sender • Link-layer solutions • hide link-related losses from sender • TCP sender may not be fully shielded • Split-connection approaches • hide any non-congestion related losses from TCP sender • since the problem is local, solve it locally

  8. End-to-End Protocols • Make the sender realize some losses are due to bit-error, not congestion. • Sender avoid invoking congestion control algorithms if non-congestion related losses occur. • E.g. Reno, New-Reno, SACK

  9. Link-Layer Protocols • Hides the characteristics of the wireless link from the transport layer and tries to solve the problem at the link layer • Uses technique like forward error correction (FEC) • Snoop, AIRMAIL(Asymmetric Reliable Mobile Access In Link-layer)

  10. Link-layer Protocols • Pros: • The wireless link is made more reliable • Doesn’t change the semantics of TCP • Fits naturally into the layered structure of network protocols • Cons: • If the wireless link is very lossy, sender times-out waiting for ACK, and congestion control algorithm starts

  11. antenna Split Connection Proposals • Split the TCP connection into two separate connections. • 1st connection: sender to base station • 2nd connection: base station to receiver • The base station simply copies packets between the connections in both directions.

  12. Split Connection • Pros: • Sender shielded from wireless link. • Better throughput can be achieved by fine tuning the wireless protocol link. • Cons: • Violates the semantics of TCP • Extra copying at the Base station.

  13. Classification of Schemes End to End Split Connection Link layer Reno SACK AIRMAIL Snoop New-Reno I-TCP M-TCP

  14. Improving TCP/IP Performance Over Wireless Networks Authors: Hari Balakrishnan, Srinivasan Seshan, Elan Amir and Randy H. Katz In Proc. 1st ACM Int’l Conf. on Mobile Computing and Networking (Mobicom), November 95.

  15. Snoop-TCP • A (snoop) layer is added to the routing code at BS which keep track of packets in both directions • Packets meant to MH are cached at BS, and if needed, retransmitted in the wireless link • BS suppress DUPACKs sent from MH to FH • BS use shorter local timer for local timeout

  16. Snoop-TCP • Changes are restricted to BS and optionally to MH as well • E2E TCP semantics is preserved

  17. Snoop Performance Poisson Distributed bit error model. Max. Bandwidth – 2Mbps

  18. Snoop connection behavior Error rate: 3.9x10-6 (A bit error every 256 Kbits on Average) Aggregate bandwidth: Snoop – 1Mbps, TCP – 0.25 Mbps Sequence numbers of the received TCP packets versus time

  19. I-TCP: Indirect TCP for Mobile Hosts Ajay Bakre, B.R. Badrinath Proceedings of the 15th International Conference on distributed Computing Systems (ICDCS '95) - 1995 IEEE

  20. I-TCP – connection setup

  21. I-TCP – LAN Performance Normal and overlapped – effective reaction to high BER.Non-Overlapped – No congestion avoidance algorithm.

  22. I-TCP – WAN Performance

  23. I-TCP • Disadvantages • End-to-end semantics is not followed • MSR sends an ack to the correspondent but loses the packet to the mobile host • Copying overhead at MSR • Conclusion • I-TCP particularly suited for applications which are throughput intensive

  24. Slow Start Receiver Sender • Sender starts by transmitting 1 segment • On receiving Ack, congestion window is set to 2. • On receiving Acks, congestion window is doubled. • Continues until Timeout occurs • After ssthresh, the sender increases its window size by 1/[current_window] on receiving Ack. (Congestion Avoidance phase)

  25. Fast Retransmission Uses Duplicate Ack to retransmit Sender Receiver Packet Loss Dup ACK 1 Dup ACK 2 Dup ACK 3 Retransmits without waiting for timeout.

  26. Fast Recovery • After Fast retransmit, perform congestion avoidance instead of slow start. • Why? • Duplicate ACK indicates that there are still data flowing between the two ends → Network resources are still available. • TCP does not want to reduce the flow abruptly by going into slow start.

  27. End to End Protocols • Tahoe: Original TCP • Slow start, Congestion avoidance, Fast retransmit • Reno: TCP Tahoe + Fast Recovery • Significant Improvement - single packet loss. • Suffers when multiple packets are dropped. • New-Reno: Reno + Stay in Fast Recovery • The first non-duplicate ACK but not the expected one. • SACK: Reno + SACK option • When multiple packets are dropped “FALL, K. AND FLOYD, S.” – “Simulation based comparisons of Tahoe, Reno, and SACK TCP” - “ACM Computer Communication Review 1996”

  28. Packet Loss Scenario • Tahoe • Fast Retransmission • ssthresh = 0.5 x current window size • congestion window = 1 • Reno, New-Reno and SACK • Fast Retransmission • Fast Recovery • congestion window = 0.5 x current window size + 3 x segment size • Increase window size by 1 on receiving a dup ACK

  29. References • Ayangolu, Paul, S., LaPorta, T., Sabnani, K., Gitlin, R. “AIRMAIL: A Link Layer Protocol for Wireless Networks,” Wireless Networks, vol. 1, pp. 47-60, 1995. • “BAKRE, A. AND BARDINATH, B. R” – “Implementation and Performance Evaluation of Indirect TCP” – “1997 - IEEE Transactions on Computers 46, 3,260–278” • “BROWN, K. AND SINGH, S” – “M-TCP: TCP for mobile cellular networks” – “1997 - Computer Communication Review, July, 19–43”

  30. References (contd.) • “H. Balakrishnan, V. N. Padmanabhan, S. Seshan, and R. H. Katz” - “A Comparison of Mechanisms for Improving TCP Performance over Wireless links” – “IEEE Trans. on Networking, vol. 5, no. 6, Dec. 1997”. • “Hari Balakrishnan, Srinivasan Seshan, Elan Amir and Randy H. Katz” – “Improving TCP/IP Performance over Wireless Networks” – “In Proc. 1st ACM Int’l Conf. on Mobile Computing and Networking (Mobicom), November 95” • “FALL, K. AND FLOYD, S.” – “Simulation based comparisons of Tahoe, Reno, and SACK TCP” - “ACM Computer Communication Review 1996, 26, 3, 5–21” • “HALA ELAARAG” - “Improving TCP Performance over Mobile Networks” – “ACM Computing Surveys, Vol. 34, N0 3, Sep 2002, pp 357-374”

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